1. Field of the Invention
The present invention relates to a liquid crystal display device which has two substrates opposed to each other via columnar spacers disposed therebetween.
2. Description of the Related Art
A liquid crystal display device is formed of two glass substrates opposed to each other with a uniform distance (gap) therebetween, and of a liquid crystal layer disposed between these substrates. In such a liquid crystal display device, in order to have a uniform gap between the two glass substrates, a plurality of spacers are formed between the two glass substrates. Spacers are classified into columnar spacers which are formed on a substrate by being patterned to extend toward the other substrate, and spherical spacers which are dispersed inside a liquid crystal layer.
FIG. 1 is a cross sectional view showing a conventional liquid crystal display device provided with columnar spacers. As shown in FIG. 1, this conventional liquid crystal display device has a glass substrate 101. Scan lines 102 are formed on the glass substrate 101, and an insulating protective film 103 is formed so as to coat the scan lines 102. Signal lines (not illustrated), pixel electrodes (not illustrated) and the like are formed on the insulating protective film 103. Another insulating protective film 104 is formed on the protective film 103 so as to coat the signal lines and the pixel electrodes. This arrangement forms TFTs (Thin Film Transistors, not illustrated) in which the signal lines are connected to the drain, the pixel electrodes are connected to the source, and the scan lines 102 are connected to the gate. This is the structure of an active matrix substrate 105.
On the other hand, another glass substrate 106 is disposed opposed to the glass substrate 101, and a black matrix (BM) 107 is formed on the surface of the glass substrate 106 that is opposed to the active matrix substrate 105. The black matrix 107 is disposed in regions including the regions corresponding to the scan lines 102 of the active matrix substrate 105. On the black matrix 107 are provided color filters (CF) 108. A protective film 109 is formed so as to coat the black matrix 107 and the color filters 108. This is the structure of a color filter substrate 110.
Inside the regions where the black matrix 107 and the color filters 108 are overlapped with each other on the color filter substrate 110, columnar spacers 111 made of acrylic resin or the like are formed to extend toward the active matrix substrate 105. The tip of each of the columnar spacers 111 is in contact with a part of the region directly above each of the scan lines 102 of the protective film 104 of the active matrix substrate 105. Between the active matrix substrate 105 and the color filter substrate 110, liquid crystal 112 is filled to form a liquid crystal layer.
However, this conventional liquid crystal display device has the following problems. When an external force is applied on the glass substrates 101 and 106, thereby dislocating the active matrix substrate 105 and the color filter substrate 110 in the direction parallel to the substrates, the frictional force between the columnar spacers 111 and the protective film 104 prevents the dislocation between the substrates from being restored even after the external force disappears. This makes the glass substrates 101 and 106 have birefringence, thereby causing light leakage regardless of the orientation of the liquid crystal. This light leakage is particularly conspicuous in black display, and is recognized as display unevenness. A reduction in the frictional force between the columnar spacers 111 and the protective film 104 by reducing the number of the columnar spacers 111 will result in plastic deformation of the columnar spacers 111, thereby causing gap unevenness.
Japanese Patent Publication No. 2002-182220 discloses a technique to reduce the frictional force between the columnar spacers and the substrate while holding the gap between the substrates. FIGS. 2 and 3 are cross sectional views showing conventional liquid crystal display devices disclosed in this patent document, Japanese Patent Publication No. 2002-182220. The liquid crystal display device shown in FIG. 2 is provided with a glass substrate 121, and with counter voltage signal lines 122 formed thereon. The counter voltage signal lines 122 are coated with an insulating film 123 on which drain signal lines 124 are formed. A protective film 125 is formed so as to coat the drain signal lines 124, and an orientation film 126 is formed on the protective film 125. This is the structure of an active matrix substrate 127. On the surface of the active matrix substrate 127, the regions including the counter voltage signal lines 122 are higher in elevation than the other regions.
On the other hand, a glass substrate 128 opposed to the glass substrate 121 has a black matrix 129 on a surface thereof that is opposed to the active matrix substrate 127. A protective film 130 is formed so as to coat the black matrix 129. On the protective film 130 are formed a plurality of columnar spacers 131a and 131b. An orientation film 132 is formed so as to coat the protective film 130 and the columnar spacers 131a and 131b. This is the structure of a color filter substrate 133. Liquid crystal 134 is filled between the active matrix substrate 127 and the color filter substrate 133 so as to form a liquid crystal layer.
In the liquid crystal display device shown in FIG. 2, the columnar spacers 131a are formed in the positions opposed to the counter voltage signal lines 122 on the active matrix substrate 127, and the tips of the columnar spacers 131a are in contact with the elevated regions on the active matrix substrate 127. On the other hand, the remaining columnar spacers 131b are formed in the positions not opposed to the counter voltage signal lines 122 on the active matrix substrate 127, and the tips of the columnar spacers 131b are not in contact with the active matrix substrate 127.
As described above, of the plurality of columnar spacers, only the columnar spacers 131a are made to be in contact with the active matrix substrate 127, whereas the columnar spacers 131b are made not to make contact. As a result, when no external force is applied on the liquid crystal display device, the frictional force is small because only the columnar spacers 131a are made to be in contact with the active matrix substrate 127, and when an external force compressing the liquid crystal display device is applied, the columnar spacers 131a are deformed to make the tips of the columnar spacers 131b come into contact with the active matrix substrate 127. This allows the gap between the substrates to be maintained.
The aforementioned patent document, Japanese Patent Publication No. 2002-182220 also discloses a technique to provide pedestal patterns directly under some of the columnar spacers, as shown in FIG. 3. In the color filter substrate 141 of the liquid crystal display device, the surface of the glass substrate 142 is provided with a black matrix 143 and color filters 144 thereon, and pedestal patterns 145 are formed on a portion of the black matrix 143. A protective film 146 is formed so as to coat the black matrix 143, the color filters 144, and the pedestal patterns 145. On the protective film 146 are formed a plurality of columnar spacers 147a and 147b. The columnar spacers 147a are formed directly above the pedestal patterns 145, whereas the columnar spacers 147b are formed in regions outside the regions directly above the pedestal patterns 145.
Consequently, the tips of the columnar spacers 147a are at a position higher than the tips of the columnar spacers 147b by the thickness of the pedestal patterns 145. Therefore, the columnar spacers 147a are in contact with the active matrix substrate (not illustrated); however, the columnar spacers 147b are not in contact with the active matrix substrate unless an external force to compress these substrates is applied. This makes it possible to maintain the gap between the substrates, while reducing the frictional force between the columnar spacers and the active matrix substrate.
However, the aforementioned conventional liquid crystal display device has the following problems. Even if the height of the columnar spacers on the color filter substrate is made uniform in the manufacture of the liquid crystal display device, the gap between the active matrix substrate and the color filter substrate varies after the liquid crystal display device is assembled. This increases a variation in luminance, chromaticity and contrast in each liquid crystal display device.